1. Field of the invention
The present invention relates to apparatus and methods for monitoring the operation of xerographic or electrophotographic copiers or printers. More particularly, the present invention relates to methods and apparatus for determining that toner is appropriately applied to photoconductive material in a xerographic copier/printer, and for providing appropriate responses to the results of such determinations. Still further, the present invention relates to the application of digital circuitry and data processing devices and processes for monitoring xerographic copier/printer operations relating to toner with appropriate control, or other responses to the results of such monitoring. The present invention is particularly useful for allowing maximum utilization of digital processes and devices in association with the operation and control of xerographic apparatus.
2. Description of the Prior Art
Electrophotographic printers and copiers appropriately discharge a previously charged photoconductor to produce an image that is transferred to a copy sheet or the like by means of toner received on the photoconductor from a developer. Acceptable quality for the final copy is a direct function of proper operation of the developer and the photoconductor itself. Degraded quality results from inadequate performance of the developer, charging elements, toner supply, photoconductor condition (i.e., aging, damage, etc ), and various other factors associated with the operation of the xerographic device. It was recognized early in the development of such machines that the light reflectance characteristics associated with the photoconductor and toned portions on that photoconductor correlate to proper operation of the machine elements.
Thus, one technique has developed in the prior art for monitoring xerographic machine operation by the process of applying a test patch of toner to the photoconductor with sensing of light reflectance from that patch for determining the amount of toner replenished into the developer. Analog circuitry for patch sensing and replenishment control is shown in U.S. Pat. No. 4,178,095 filed Apr. 10, 1978 by J. R. Champion and S. D. Seigal, as well as in the April 1977 IBM TECHNICAL DISCLOSURE BULLETIN at pages 4078-4079 in the article entitled "Toner Concentration Control" by G. L. Smith. Conversion of the analog signal produced from monitoring a test patch into digital signals for control by a computer in a xerographic machine is also well known in the prior art. Such systems generally monitor other factors such as the toner in the developer sump as well as continuous test patch monitoring until an appropriate result is produced.
An example of digital circuitry interfacing between the analog patch sensor photodetector system and a controlling computer is shown in FIG. 4 of U.S. patent application Ser. No. 291,136 filed Aug. 7, 1981 by L. M. Ernst, now U.S. Pat. No. 4,337,338.
A typical prior art approach is to perform light reflectivity sampling against both a clean photoconductor and a toned patch on the photoconductor, with the patch either in the normal image area or in the photoconductor area outside of the image area A test sample of clean and toned patches are initially performed and the results stored for comparison against later such test results. Differences from the standard and later test results are used to control the rate of toner replenishment into the developer from a reservoir. The rate of replenishment is increased if the toned patch reflectance test result shows a light test patch, while the rate is decreased if excessively dark test patches are obtained. Other electrophotographic process adjustments are available in response to the toned test patch result such as variations in illumination level, adjustments to biasing for the developer and/or coronas and the like, as well as variations in replenisher rate.
Unfortunately, the results of a particular toned patch reflectance test do not necessarily indicate the operating condition of the electrophotographic components or the condition of the photoconductor itself. Where a photodetector result is analog-to-digital converted, the resulting digital signal actually indicates the level of a small area sample rather than an indication of the overall quality of the complete patch. Accordingly, any aberration associated with the sample increment can produce corrective efforts on the part of the controlling computer which are inappropriate for the average condition of the machine and photoconductor. The present invention overcomes the shortcomings of the prior art by advantageously utilizing digital circuitry and controlling computers to ensure accuracy of toner level sampling as well as to allow corrective action by the controlling computers or microprocessors, as appropriate.